1. Field of the Invention
The present invention relates to an image processing circuit for use in a digital full-color electrographic copying machine having a forgery preventing function.
2. Description of the Prior Art
In recent years, digital color electrographic copying machines have been spreading, and most of them are used for beneficial purposes. However, since anyone can easily obtain a copy with a high resolution and an excellent color reproducibility because of the improvement in performance of such copying machines, the forgery of bank notes, which is a serious crime, may easily be committed.
For this reason, manufactures of copying machines of this type enables tracing with respect to a copy (i.e. the tracing of the copying machine with which the original is copied and of the date of the copying) in order to prevent to the utmost the forgery of bank notes which is a crime exerting a great influence on society. Specifically, when an original is copied, tracing information such as the machine number of the copying machine and the date of the copying are added to the copy of the original. In order that the image other than the added information is not damaged due to the addition of such tracing information, it is necessary to add the information at a density where the information cannot visually be recognized. For this reason, a tracing information adding method is employed using a yellow color image data which is difficult to visually recognize.
In such a tracing information adding method, when tracing information is added by adding a yellow color image data of a predetermined density (this density should be as low as possible so that visual recognition is impossible), for example at a low density portion (white portion) where there is hardly any yellow color image data, the density of the added tracing information is too low, so that it may be impossible to read out the information from forged bank notes by a color scanner. Therefore, a way to solve this problem is desired.
In the above-described method of adding tracing information, in order to further decrease the influence on the reproduced image by the addition of the tracing information, it is desired that the tracing information is added uniformly to the entire image. In this case, the ratio of the area of the added tracing information to the area of the reproduced image is necessarily fixed. That is, the ratio of the area of the tracing information to the area of the reproduced image is necessarily fixed even if the contents of the tracing information are changed, for example, even if the copy date is changed every day.
Further, it is desirable that the tracing information added in this manner should be easily and surely detected by anyone that has no special skills so that the tracing of the copy may be made rapidly at the sites where criminal cases occur.